无法分配给属性：“自身”是不可变的

我正在创建一个页面视图应用程序。我已经为ViewController编写了代码：

import UIKit

class ViewController: UIViewController {

var pageViewController : UIPageViewController!
var pageTitles : NSArray!
var pageImages : NSArray!

@IBAction func restartAction(sender: AnyObject) {}

override func viewDidLoad() {
    super.viewDidLoad()

    self.pageTitles = NSArray(objects: "This Is Page One!", "This Is Page Two!")
    self.pageImages = NSArray(objects: "page1", "page2")

    self.pageViewController = self.storyboard?.instantiateViewControllerWithIdentifier("PageViewController") as! UIPageViewController

    self.pageViewController.dataSource = self // <–– Error

    let startVC = self.viewControllerAtIndex(0) as ContentViewController
    let viewControllers = NSArray(object: startVC)

    self.pageViewController.setViewControllers(viewControllers as? [UIViewController], direction: .Forward, animated: true, completion: nil)

    self.pageViewController.view.frame = CGRectMake(0, 30, self.view.frame.width, self.view.frame.height - 60)

    self.addChildViewController(self.pageViewController)
    self.view.addSubview(self.pageViewController.view)
    self.pageViewController.didMoveToParentViewController(self)

}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

func viewControllerAtIndex(index: Int) -> ContentViewController {
    if ((self.pageTitles.count == 0) || (index >= self.pageTitles.count)) {
        return ContentViewController()
    }

    let vc : ContentViewController = self.storyboard?.instantiateViewControllerWithIdentifier("ContentViewController") as! ContentViewController

    vc.imageFile = self.pageImages[index] as! String
    vc.titleText = self.pageTitles[index] as! String
    vc.pageIndex = index

    return vc
}



func pageViewController(pageViewController: UIPageViewController, viewcontrollerBeforeViewController viewController: UIViewController) ->UIViewController? {

    let vc = viewController as! ContentViewController
    var index = vc.pageIndex as Int

    if index == 0 || index == NSNotFound {
        return nil
    }

    index--
    return self.viewControllerAtIndex(index)

}

func pageViewController(pageViewController: UIPageViewController, viewcontrollerAfterViewController viewController: UIViewController) ->UIViewController? {

    let vc = viewController as! ContentViewController
    var index = vc.pageIndex as Int

    if index == NSNotFound {
        return nil
    }

    index++

    if index == self.pageTitles.count {
        return nil
    }

    return self.viewControllerAtIndex(index)

}

}

但是我在这条线上遇到了错误self.pageViewController.dataSource = self。错误是：

无法分配给属性：“自身”是不可变的

我不确定为什么自我是一成不变的，以及如何解决它。如果有人可以帮助，请提前感谢。